1. Field of the Invention
The invention relates to transducers, and more particularly to a plurality of alternately layered transducing elements and acoustical lenses for use in focussing acoustical waves at different colinear regions without any mechanical motion.
2. Description of the Prior Art
Ultrasound is used to examine inside a specimen and produce an image. An ordinary transducer is electrically pulsed, and in response it transmits an ultrasonic (mechanical) wave. The wave passes through an acoustic lens to focus at a particular location inside the specimen. That location is determined by the focal length of the lens. The wave interacts with the specimen producing echoes, some of which reflect back onto the lens and through to the transducer. The transducer then produces electrical signals, and those which correspond to the echoes of the field are used to make an image. A highly focussed wave yields good resolution but only over a small depth of field. The quality of resolution and depth of field are inversely related according to standard lens properties.
To focus over a large depth of field, two or more lenses of different focal lengths can be interchanged mechanically to yield two or more fields. The pulse-echo procedure would be repeated for each lens whereby the echoic electrical signals corresponding to the fields would be combined to effect a large depth of field. However, interchanging lenses takes too long and requires precise alignment. The invention provides a large depth of field without any mechanical motion.